Use the cstr function to convert numeric values into text which can then be concatenated into your string. Test for null values and replace them with a valid character or string.
Also make sure that the value you want to retrieve from the combo control is the bound value as you have discovered. If it is not, then you will also need to specify the column number (numbering starts at zero). Therefore [Combo2](1) should retrieve the value from the second column of your control.